[28.x backport] Add escape hatch for GODEBUG=x509negativeserial

cli/command/cli.go

@@ -282,6 +282,17 @@ func (cli *DockerCli) Initialize(opts *cliflags.ClientOptions, ops ...CLIOption)
 	}
 	filterResourceAttributesEnvvar()
 
+	// early return if GODEBUG is already set or the docker context is
+	// the default context, i.e. is a virtual context where we won't override
+	// any GODEBUG values.
+	if v := os.Getenv("GODEBUG"); cli.currentContext == DefaultContextName || v != "" {
+		return nil
+	}
+	meta, err := cli.contextStore.GetMetadata(cli.currentContext)
+	if err == nil {
+		setGoDebug(meta)
+	}
+
 	return nil
 }
 
@@ -475,6 +486,57 @@ func (cli *DockerCli) getDockerEndPoint() (ep docker.Endpoint, err error) {
 	return resolveDockerEndpoint(cli.contextStore, cn)
 }
 
+// setGoDebug is an escape hatch that sets the GODEBUG environment
+// variable value using docker context metadata.
+//
+//	{
+//	  "Name": "my-context",
+//	  "Metadata": { "GODEBUG": "x509negativeserial=1" }
+//	}
+//
+// WARNING: Setting x509negativeserial=1 allows Go's x509 library to accept
+// X.509 certificates with negative serial numbers.
+// This behavior is deprecated and non-compliant with current security
+// standards (RFC 5280). Accepting negative serial numbers can introduce
+// serious security vulnerabilities, including the risk of certificate
+// collision or bypass attacks.
+// This option should only be used for legacy compatibility and never in
+// production environments.
+// Use at your own risk.
+func setGoDebug(meta store.Metadata) {
+	fieldName := "GODEBUG"
+	godebugEnv := os.Getenv(fieldName)
+	// early return if GODEBUG is already set. We don't want to override what
+	// the user already sets.
+	if godebugEnv != "" {
+		return
+	}
+
+	var cfg any
+	var ok bool
+	switch m := meta.Metadata.(type) {
+	case DockerContext:
+		cfg, ok = m.AdditionalFields[fieldName]
+		if !ok {
+			return
+		}
+	case map[string]any:
+		cfg, ok = m[fieldName]
+		if !ok {
+			return
+		}
+	default:
+		return
+	}
+
+	v, ok := cfg.(string)
+	if !ok {
+		return
+	}
+	// set the GODEBUG environment variable with whatever was in the context
+	_ = os.Setenv(fieldName, v)
+}
+
 func (cli *DockerCli) initialize() error {
 	cli.init.Do(func() {
 		cli.dockerEndpoint, cli.initErr = cli.getDockerEndPoint()

cli/command/cli_test.go

@@ -18,6 +18,7 @@ import (
 
 	"github.com/docker/cli/cli/config"
 	"github.com/docker/cli/cli/config/configfile"
+	"github.com/docker/cli/cli/context/store"
 	"github.com/docker/cli/cli/flags"
 	"github.com/docker/docker/api"
 	"github.com/docker/docker/api/types"
@@ -353,3 +354,23 @@ func TestHooksEnabled(t *testing.T) {
 		assert.Check(t, !cli.HooksEnabled())
 	})
 }
+
+func TestSetGoDebug(t *testing.T) {
+	t.Run("GODEBUG already set", func(t *testing.T) {
+		t.Setenv("GODEBUG", "val1,val2")
+		meta := store.Metadata{}
+		setGoDebug(meta)
+		assert.Equal(t, "val1,val2", os.Getenv("GODEBUG"))
+	})
+	t.Run("GODEBUG in context metadata can set env", func(t *testing.T) {
+		meta := store.Metadata{
+			Metadata: DockerContext{
+				AdditionalFields: map[string]any{
+					"GODEBUG": "val1,val2=1",
+				},
+			},
+		}
+		setGoDebug(meta)
+		assert.Equal(t, "val1,val2=1", os.Getenv("GODEBUG"))
+	})
+}